Recently, as the electron tubes such as projection tube, HDTV and projection television tend to become larger, new type of cathodes are to be adapted to such an electron tube are demanded. The cathodes suitable to this demand should include a dispense cathode having a higher current density than that of oxide cathode and having a longer lifetime, for which steady research and development have been carried out. These dispenser cathodes are divided into an impreganated type and a cavity reservoir type. However, these dispenser cathodes have an operating temperature of 900.degree. to 1100.degree. C. which is about 200.degree. C. higher than that of a conventional oxide cathodes. Such a high operating temperature requires the cathode to adopt heater having a large calorimeter, so that the cathode itself and other parts neighboring thereto should be made of a heat-resistant material. Further, a higher operating temperature increases the amount of Ba (or BaO) evaporated from the cathode. So Ba evaporated is attached to the neighboring parts, especially attached to a control grid positioned adjacently to the cathode, so that a second emission, so called a grid emission, damaging the cathode is generated. The cathode improving the defect which is caused by a higher operating temperature of this cathode include "M-type" dispenser cathode disclosed in U.S. Pat. No. 3,373,307 and "Sc-type" impregnated cathode disclosed in U.S. Pat. No. 4,737,639.
"M-type" dispenser cathode comprises a porous tungsten base body coated with a platinum group element such as Os, Ir, Re. Ru having a work function higher than that of tungsten in which, Ba concentration on the cathode surface is enhanced due to the material coated on the surface of the tungsten base body, thereby reducing a work function.
In "Sc type" dispenser cathode, a layer containing Sc is coated on the surface of the porous base body in order to reduce the operating temperature. However, these dispenser cathodes have an operating temperature of about 100.degree. to 200.degree. C. higher than that of the conventional oxide cathode, therefore there are problems of the selection of the material as described above, of the short lifetime by the evaporation of thermoelectron emission material, and of long time required in aging.